Various methods are known for fabricating patterns in polymer structures. For example, molding techniques such as injection molding and compression molding are commonly employed methods for making patterned polymer structures. It is often desirable to fabricate a pattern in a polymer structure (or layer) that includes through holes. Fabrication of through holes by molding often provides unsatisfactory results. In particular, the through holes may be partially or even completely blocked by undesired material (sometimes referred to as “flash”). This problem with through holes has been recognized and addressed in the literature. For example, U.S. patent application 2004/0178537 considers injection molding where pins for forming through holes are longer than the cavity depth. Thus the pins are under compression, and the tendency of excess material to remain at through holes is reduced. Alternatively, post-processing is frequently employed to remove the undesired material.
Generally, the problems associated with through hole fabrication become more acute as the size of through holes decreases and as the density of through holes increases. Approaches that are suitable for fabricating a single large through hole (e.g., punching a single hole with a pin) tend to be completely unsuitable for fabricating many closely spaced small through holes, especially in thin layers which can require careful handling to avoid mechanical damage. For example, injection molding (as in U.S. 2004/0178537 above) requires careful attention to detail in order to avoid formation of voids or trapped air bubbles in a polymer film as a result of injection into the mold. As the through hole size decreases and/or as through hole density increases, the difficulties of injection molding tend to increase. Similarly, post-processing to remove excess material from through holes is an approach which is less and less attractive as the hole size decreases, since damage to through holes by post-processing is increasingly difficult to avoid as the hole size decreases.
Patterned polymer layers and structures are under active investigation for various biological and/or medical applications. For example, U.S. patent application 2002/0182241 considers three dimensional polymer scaffolds formed by bonding two-dimensionally patterned polymer layers together. In this work, various patterning methods for polymer layers including through holes are mentioned (e.g., casting, stamping and embossing), but the specific difficulties associated with through hole fabrication are not considered.
Further examples in the art of polymer fabrication methods include U.S. Pat. No. 6,598,701, U.S. 2004/0044111, and U.S. Pat. No. 4,752,352. Although these references consider various aspects of polymer fabrication, none of these references provides a method suitable for forming a large number of densely spaced, small, flash-free through holes.
Accordingly, it would be an advance in the art to provide a method of patterning a polymer layer with a pattern including a large number of densely spaced, small, flash-free through holes.